| In the early 1950's, the super-exchange, double-exchange and electrical transport properties of transition-metal oxides with perovskite structure had been extensively investigated, but until the last decades, after the discovery of CMR(colossal magnetoresistance) effect, these materials renewedly became a hot subject of research for the material physics and condensed matter physics. This is not only due to their potential technology applications in various magnetic devices but also the intriguing physical properties as a kind of strongly correlated electron systems such as metal-insulator transition induced by applied field, orbital ordering, charge ordering, and phase separation etc.. So understanding of the microscopic physics underlying the CMR properties is extraordinarily important.In this thesis we focus on investigating the structure and the electromagnetic behaviors of the grain boundaries. We hope the low field magnetoresistance(LFMR) can be enhanced, especially at the room temperature, by intentionally changing the conformation of grain boundaries in future material applications. The thesis can be divided into four chapters.In the first chapter, we briefly review the research progress of the CMR materials and the physical properties of perovskite-structured manganites, including crystalline structure, electronic configuration, magnetic property and electronic transport etc.. we expatiate the physical mechanisms about CMR, such as double-exchange, Jahn-Teller effect, especially the phase separation phenomena which have been widely investigated in recent years, and the low field magnetoresistance caused by grain boundaries. Moreover, we make a brief presentation about the magnetostriction in rare-earth manganites.In the second chapter, we discuss the influence of A-site cation average size〈rA〉and size variance factorσ2 on structural, magnetic and electrical properties of La1-xAxMnO3(A=Ca, Sr, Ba). As a result, La0.7Ba0.3MnO3 polycrystalline sample exhibits discriminating properties with La0.7Ca0.3MnO3 and La0.7Sr0.3MnO3, for example, its metal-insulator transition (MIT) temperature TP is much less than the Curie temperature TC, and possesses big residual resistance. By SEM, the grain size of La0.7Ba0.3MnO3 polycrystalline is much less than the two others, existing obvious grain boundaries. Thus, in La0.7Ba0.3MnO3 polycrystalline, grain boundaries have largest influence on its transport properties.In the third chapter, we prepare La0.7Ba0.3MnO3 polycrystalline and film samples, and discuss the influences of grain boundaries by comparing the large discrepancies of electromagnetic transport properties between them. Polycrystalline sample display a larger resistivity and a lower metal-insulator transition temperature, which could be attributed to the grain boundary contribution. Using an appropriate model, we obtain the temperature dependence and magnetic dependence of grain boundary resistivity of La0.7Ba0.3MnO3 polycrystalline sample, and find a novel positive magnetoresistance phenomenon. We argue that the grain boundary is a phase separation system rather than a homogonous phase. With decreasing temperature approaching its Curie temperature, the FM clusters appear in grain boundaries, and form percolation channels. The special conformation of grain boundaries determines that the giant negative volume magnetostriction in La0.7Ba0.3MnO3 compound has big influence on transport properties. At last, we simulate and calculate the transport properties in grain boundaries, and the result confirm that the low-resistive FM channels formed because of phase separation are crucial to the transport properties of grain boundaries.The fourth chapter gives a conclusion for the whole thesis and put forward some questions awaited for further research.
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